Highly Selective and Sensitive Thiocyanate PVC Membrane ...Highly Selective and Sensitive...

Int. J. Electrochem. Sci., 6 (2011) 3178 - 3195

International Journal of

ELECTROCHEMICAL SCIENCE

www.electrochemsci.org

Highly Selective and Sensitive Thiocyanate PVC Membrane

Electrodes Based on Modified Zeolite ZSM-5

Atefeh Badri*, and Pouya Pouladsaz

Department of Chemistry, Islamic Azad University, Shahreza Branch, P.O. Box 311-86145, Shahreza,

Isfahan, Iran *E-mail: [email protected]

Received: 12 June 2011 / Accepted: 11 July 2011 / Published: 1 August 2011

A highly selective membrane electrode based on surfactant modified zeolite is presented. The

proposed electrode shows very good selectivity for thiocyanate ions over a wide variety of common

inorganic and organic anions. The sensor displays a near Nernstian slope of −58.9 ± 0.9 mV per

decade. The working concentration range of the electrode is 1.00×10-6- 1.10×10-1 M with a detection

limit of 8.33×10−7 M . The response time of the sensor in whole concentration ranges is very short

(

Int. J. Electrochem. Sci., Vol. 6, 2011

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Thiocyanate finds many industrial applications and though not as toxic as cyanide, it is harmful

to aquatic life. Its determination at low levels in water and industrial effluents is therefore important.

Thiocyanate sensors are potentially useful for detecting this compound in biological samples (e.g.

urine and saliva), where elevated levels of thiocyanate correlate with excessive cigarette smoking. If

the content of thiocyanate ion is a little higher in the body than normal, the protein dialysis will be

affected and it may even result in coma. Due to the urgent need for selective potentiometric

determination of trace amounts thiocyanate ion, especially in food, biological and water samples, many

coordination compounds have been employed as an ionophore in the construction of ISEs thiocyanate

ion [2–10].

Recent decade scientific researches introduced several PVC based sensors for different anions,

such as I− [11–13], SCN- [3, 4, 9], SO4 2− [14–19], ClO4

− [20], NO2− [21, 22].

In this work, we report the use of a surfactant-modified zeolite ZSM-5 as an excellent

ionophore for the preparation of a highly selective and sensitive polymeric membrane sensor for

thiocyanate ion. The proposed sensor resulted in selectivity coefficients superior to those reported for

other thiocyanate-selective electrodes.

2. EXPERIMENTAL

2.1. Reagents and chemicals

Sodium silicate used was the product of Merck and its composition is SiO2 (25.2-28.5%), Na2O

(7.5-8.5%), H2O (63%). Chloridric acid (37%), sulfuric acid (98%), sodium hydroxide (98%),

tetrapropylammonium bromide (TPABr 98%), Aluminum sulfate eighteen hydrate (>99.3%), sodium

chloride (99.5%) were supplied from Merck.

The polymeric matrix was prepared from high relative molecular weight polyvinyl chloride

(PVC). Reagent grade dioctylphthalate (DOP), tetrahydrofuran (THF) and

hexadecyltrimethylammonium bromide (HDTMABr) were purchased from Aldrich or Fluka and used

as received. Sodium and potassium salts of all anions were of the highest purity available (all from

Merck) and used without any further purification. The pH of solutions was adjusted with sodium

hydroxide or hydrochloric acid solution as appropriate. All other reagents used were of analytical

reagent grade. De-ionized water was used throughout the experiments. A 0.1M stock solution of

thiocyanate was prepared by dissolving an appropriate, accurate amount of KSCN. The standard

1.0×10−7–1.0×10−1M solutions of thiocyanate were prepared daily by sequential dilution of the

appropriate stock solution with doubly distilled water.

2.2. Synthesis and modification of ZSM-5 zeolite

2.4984 g (0.0427 mol) of sodium chloride and 0.5931 g (0.00089 mol) of hydrated aluminum

sulfate, were dissolved in 10.1200 g (0.5622 mol) of distilled water. 1.8945 g (0.00712 mol)

tetrapropylammonium bromide, 7.1200 g (0.3955 mol) of distilled water and 1.0883 g (0.01088 mol)


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sulfuric acid were added with vigorous stirring. 15.0000 g (0.07125 mol SiO2) of sodium silicate

solution was subsequently added and the mixture was stirred for 60 min. The pH of the mixture was

9.38. The mixture was transferred into the autoclave. The autoclave was kept into an oven at 110 °C

for about 2 h. Then, the temperature was increased to 230 °C and kept at that level for another 5 h. The

solid phase obtained was filtered out, washed with water several times, dried at 110 °C for about 12-14

h and subsequently calcined at 540 °C for 3 h [23].

For preparation of surfactant modified zeolite (SMZ), the Na-ZSM-5 zeolite (2 g) was mixed

with 100 mL of 50 mM HDTMA solutions, and stirred for 24 h on a magnetic stirrer. The mixture was

then centrifuged at 5000 rpm for 20 min and the resulting SMZ was dried in air.

2.3. Characterization of the synthesized zeolite and the SMZ

The X-ray diffraction patterns of zeolite ZSM-5 was determined using a Bruker diffractometer,

D8ADVANCE, X-ray tube anode: Cu, wavelength: 1.5406 (Cu Kα radiation), filter: Ni. Fourier

transformation infrared (FT-IR) spectra of samples, on KBr pellets, were recorded with a Nicolet

single beam FT-IR (Impact 400D) spectrometer in the range of 400-4000 cm-1. All samples were

stored in a desiccator until the FT-IR analysis and KBr of spectrometry grade was also heated at 25 ◦C

to remove the adsorbed water before pressing. Then 1mg dry powdered sample was mixed with 100

mg KBr, followed by compressing the mixture to pellets with 12 mm diameter and investigating by the

FT-IR spectrometer.

2.4. Electrode preparation

The procedure to prepare the best PVC membrane was to mix thoroughly 25.5 mg of powdered

PVC, 10.0 mg of SMZ as ionophore and 64.5 mg of plasticizer DOP. Then the mixture was dissolved

in 3 mL of dry THF. The resulting clear mixture was evaporated slowly until an oily concentrated

mixture was obtained. A Pyrex tube (5 mm i.d.) was dipped into the mixture for about 10 s so that a

nontransparent membrane of about 0.3 mm thickness is formed. The tube was then pulled out from the

mixture and kept at room temperature for about 2 h. The tube was then filled with internal solution

(1.0×10-2 M KSCN). The electrode was finally conditioned for 16 h by soaking in a 1.0×10-2 M KSCN

solution. A silver/silver chloride electrode was used as an internal reference electrode. The electrode

was instantly ready for use.

2.5. Potentiometric measurements

The potentiometric measurements were performed with a Metrohm pH meter 691 pH/Ion

Meter (Metrohm) at 25.0 ± 1C. A SMZ-membrane thiocyanate-selective PVC electrode and a

standard Ag/AgCl electrode shielded by an intermediate salt-bridge compartment, containing the

background electrolyte in order to prevent any transfer of potassium ions into the measuring solution,

were used as an indicator electrode and an external reference electrode, respectively. All potentials


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were reported versus this reference electrode. The cells for millivolt measurements were of the

following type:

Ag/AgCl, KCl (3M) | internal solution KSCN (1.0 × 10-2 M) |electrode membrane | test solution

| Ag/AgCl, KCl (3M)

The potential reading of each solution was recorded when it became stable, and then plotted as

a logarithmic function of SCN- anion concentration. The activities of anions were based on the

activity-coefficient data () calculated from the modified form of Debye-Huckel Equation [24], which

is applicable to any ion and is as follows:

(1)

where is the ionic strength and Z is the valency. All measurements were carried out at room

temperature. The pH measurements were performed using a corning model 125 pH meter equipped

with a combined electrode.

3. RESULTS AND DISCUSSION

3.1. Synthesized zeolite and SMZ characterization

3.1.1. X-ray diffraction studies

Synthesized ZSM-5 zeolite was characterized by X-ray diffraction.

0

2000

4000

6000

6 10 20 30 40 50 60

Figure 1. XRD patterns synthesized zeolite ZSM-5.


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Figure 1 shows representative XRD patterns of sample synthesized. The measurements were

conducted in continuous /2 scan refraction mode. The anode was operated at 30 KV and 15 mA the

2 angles were measured 5–60 at the rate of 2.min-1. The characteristic lines at 2θ of 7.9, 8.8, 23.1,

23.8 and 24.5 are observed from XRD pattern that show a good agreement with the data of Na- ZSM-5

zeolite [25]. The crystallinity of the prepared sample was calculated using the ratio of the sum of the

areas of the most intense peaks for zeolite ZSM-5 sample (in above mentioned 2θ values) to the same

peaks for the standard (zeolite ZSM-5) and multiplying by 100. This analysis shows that the product

has a typical zeolite ZSM-5 structure with no amorphous material.

3.1.2. FT-IR studies

FT-IR lattice vibration spectra were used to investigate the influence of surfactant on the

zeolite framework.

Figure 2. FT-IR patterns for (a) HDTMA (b) synthesized ZSM-5 and (c) SMZ.


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Representative spectra of zeolite HDTMA, ZSM-5 and SMZ in the range of 400–4000cm−1 are

shown in Fig. 2. According to Fig. 2b, the observed frequencies at 3436, 1623, 1098 and 551cm−1

agrees with the infrared spectral data, which has been obtained for zeolite ZSM-5 by Flanigen et al.

[26]. The IR pattern of SMZ-ZSM-5 shows characteristic peaks at 2919, 2856, 1478 and 687 cm–1 in

fig. 3c, which indicate the incorporation of HDTMA into the zeolite structure. There is a slight shift in

peaks at each wavenumber, however, both of the patterns are nearly the same. Specially, the prescence

of peaks at 2919, 2856 and 1478 cm-1 in SMZ spectrum (related to C-H, C-C and N-C vibrations in the

surfactants, respectively), that are absent in the Na- ZSM-5 spectrum, confirm loading of HDTMA on

zeolite surface.

3.2. Theoretical considerations of SMZ and electrode behavior

As previously mentioned, the SMZ can be used for sorption of anionic species. This is based on

the conceptual model which was illustrated graphically in the previous work [27]. At low

concentrations, HDTMA forms a monolayer on the zeolite surface with the hydrophobic ends of the

molecules turned toward the solution. With increasing coverage, a second HDTMA layer is also

formed. This bilayer structure causes arrangement of positively charged functional groups toward the

solution, which can serve as sorption sites for anions [28]. Complete formation of the second layer,

means that sorbed HDTMA concentration is twice of the external cation exchange capacity (ECEC).

Preliminary experiments showed that when SMZ is mixed with a solution containing

thiocyanate anion, the concentration of thiocyanate in the remaining solution is decreased. Therefore,

we were prompted to use the SMZ as an active ingredient for construction of a potentiometric sensor

for this anion. The sorption mechanism can be shown by the following reaction:

R4N+ in SMZ + ClO4

− R4N+ ClO4

− (2)

where R4N+ represents the anion-exchange sites of HDTMA surfactant on SMZ. According to

the above discussions, when proposed electrode is immersed in thiocyanate solutions, anion exchange

reaction takes place between anions on the electrode surface (Br−) and ClO4− in solution.

3.3. Optimization of the membrane composition

In preliminary studies, some experiments regarding the response towards mono and divalent

anions were carried out. Except with SCN- ion, the E versus pM plots is nonlinear within the

concentration range of 1.0×10−1 to 1.0×10−7 M, and exhibit hypo-Nernstian slopes (table 1). Our

preliminary experiments showed that the membrane prepared by using the zeolite ZSM-5 as ion

sensing proved to be suitable one, and all of optimizations were done on SMZ-PVC electrode. First,

the amount of SMZ modifier was optimized. The responses of the electrodes to several SMZ

compositions are presented in Table 2. As can be seen, only the electrode containing 10% SMZ which

was prepared from Na–ZSM-5 at 50 mM HDTMA, as described in section 2.2, showed Nernstian


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behavior. Electrodes which were prepared at 100 and 200 mM HDTMA solution do not show a

Nernstian behavior.

Table 1. Potentiometric response of the electrode toward different anions (n=4).

Anion Slope(mV.decade-1) r2 Linear range (-log C)

ClO4- 46.7 ± 0.8 0.9809 ± 0.08 5.2-1.1

ClO3- 36.1 ± 0.9 0.9903 ± 0.08 5.0-3.1

SO42- 19.2 ± 1.1 0.9393 ± 0.04 4.8-2.1

SO32- 20.2 ± 0.9 0.9807 ± 0.07 5.1-1.9

C2O42 17.1 ± 0.9 0.9866 ± 0.04 4.8-1.9

I- 41.2 ± 0.8 0.9791 ± 0.08 5.1-2.6

NO3- 18.1 ± 0.8 0.9870 ± 0.008 4.1-2.1

HPO42- 30.4±1.4 0.9678 ± 0.009 5.1-2.0

SCN- 60.7 ± 1.6 0.9933 ± 0.005 6.6-1.2

Br- 34.3 ± 0.9 0.9661±0.004 4.8-2.0

Table 2. Optimization the modifier amount.

Electrode Modifier percentage Slope (mV. decade-1) (n=5)

A 4%(NaZSM-5+100 mM HDTMA) -42.0 ± 1.1

B 10%( NaZSM-5+100 mM HDTMA) -53.1 ± 0.5

C 15%( NaZSM-5+100 mM HDTMA) -45.1 ± 0.7

D 4%( NaZSM-5+200 mM HDTMA) -42.5 ± 0.8

E 10%( NaZSM-5+200 mM HDTMA) -53.1 ± 0.5

F 15%( NaZSM-5+200 mM HDTMA) -56.1 ± 0.8

G 4%( NaZSM-5+50 mM HDTMA) -40.1 ± 1.1

*H 10%( NaZSM-5+50 mM HDTMA) -58.7 ± 0.3

I 15%( NaZSM-5+50 mM HDTMA) -41.1 ± 0.8

Table 3. Composition and potentiometric characterization of thiocyanate selective electrode based on

SMZ-PVC membrane (n=4).

No PVC% SMZ% DOP% Avrage Slope

(mV.decad-1)

r2 Linear

range(-log

C)

RSD

1 29 4.0 67 53.4±1.2 0.9910 ± 0.0030 5.5-1.0 3.2

*2 25.5 10.0 64.5 58.6±0.9 0.9960 ± 0.0190 6.0-1.1 1.7

3 23.5 15.0 61.5 44.3±1.3 0.9711 ± 0.0200 4.4-1.1 4.3

4 21 20.0 59 39.2±0.8 0.9701 ± 0.0141 3.9-1.2 2.9

In this case, a partially formed bilayer may be loaded on the zeolite surface. Consequently, the

obtained SMZ does not respond appropriately to SCN–. This completely agrees with the results which


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were obtained by Wingenfelder et al. [29] for antimonate sorption studies. They found that, in the

treatments about two times of the ECEC, the zeolite took up over 98% of the initial HDTMA in

solution. When the applied HDTMA concentrations were over two times as high as the ECEC, the

zeolite took up 84% of the initial HDTMA concentration.

The potential response of a sensor is greatly related to the membrane ingredients [30-34]. The

ratio of membrane ingredients were optimized so that the membranes develop reproducible, noiseless

and stable potentials. Composition of the membranes, which resulted in the best performance, is shown

in Table 3 along with their potentiometric characteristics. As it can be seen, electrodes made from

membranes containing 10% (w/w) zeolite in the presence of 25.5% polymer, 64.5% dioctylphthalate

were found to be the optimum one in the development of this sensor (membrane no. 2, Table 3). Thus,

this membrane composition was used for subsequent works.

3.4. Effect of internal reference solution

The concentration of the internal solution (KSCN) in the electrode was changed from 1.0×10−2

to 1.0 ×10−4 M and the emf-pSCN− plots were obtained and the results are shown in Fig. 3.

Figure 3. Effect of concentration of internal reference solution on the electrode response at various

thiocyanate concentrations in the range of 1.0×10−1 M-1.0×10-3 M.

It was found that the variation of the concentration of the internal KSCN solution did not cause

any significant difference in potential response except for an expected change in the intercept of the

resulting plots. A 1.0×10−2 M concentration of internal reference solution is quite appropriate for


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smooth functioning of the electrode system. Then conditioning time was studied by changing the time

of being electrode in conditioning situation (Table 4). The obtained results showed the best Nernstian

behavior was for 16 h.

Table 4. Conditioning time results for SMZ-PVC membrane electrode (10% modifier; -logC = 6 - 2; )

n=5).

r 2 Slope (mV.decade-1) t (h)

0.9893 ± 0.0377 38.0 ± 0.9 4

0.9877 ± 0.0438 39.9 ± 1.6 6

0.9699 ± 0.0069 40.9 ± 0.9 8

0.9899 ± 0.0066 46.7 ± 0.4 10

0.9901 ± 0.0038 49.9 ± 0.9 12

0.9870 ± 0.0080 59.3 ± 0.9 *16

0.9982 ± 0.0723 52.4 ± 0.9 18

0.9191 ± 0.0132 53.8 ± 1.2 24

0.9694 ± 0.0066 55.5 ± 0.9 48

3.5. Effect of pH

The influence of pH of the test solution (1.0×10−3 M thiocyanate) on the potential response of

the SMZ-membrane sensor was tested in the pH range 1.1–13.1 (adjusted with either HCl or NaOH)

and the results are shown in Fig. 4.

Figure 4. Influence of pH on the potential response of the thiocyanate-selective electrode to 1.0×10-2

M SCN-.


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As seen, the membrane electrode can be suitably used in the pH range 2.0–10.1. A pH of 6.0

was adjusted with 0.1 mol L−1 acetic acid-acetate buffer was used for further studies. However, drastic

potential changes are observed at higher and lower pH values. This is most probably due to the

simultaneous response of the electrode to SCN− and OH−, at pH > 10.1, and some ionophore

protonation and consequent diminished responsivity to SCN− and/or simultaneous response to

thiocyanate and chloride ions, at pH < 2.0.

3.6. Effect of ionic strength on the response of the electrode

The effect of ionic strength (1.0×10−4 to 3.4×10-3 M NaNO3) on the calibration curve of the

SMZ-PVC electrodes was investigated and the obtained results are listed in Table 5.

Table 5. Effect of ionic strength on optimized SMZ-PVC membrane electrode response.

Concentration of

NaNO3 (M)

Slope= x (n=6) (mV.decade-1)

texp Linear range of thiocyanate

(M)

Intercept

(mV)

1.0×10-4 -59.1±0.56 0.0.40 1.00×10-6-1.10×10-1 236

3.0×10-4 -59.5±0.76 1.18 1.00×10-6-1.10×10-1 235

4.9×10-4 -59.3±0.61 0.73 1.00×10-6-1.10×10-1 233

1.0×10-3 -59.5±0.66 0.95 1.00×10-6-1.10×10-1 232

2.0×10-3 -59.7±0.78 1.43 1.00×10-6-8.97×10-2 197

3.4×10-3 -59.3±0.67 0.33 1.00×10-6-8.94×10-2 190

Note: t 0.05, 5 =2.57 (Ref. 35)

The electrode response has a Nernstian slope with linear range of 1.00×10-6-1.10×10-1 of

thiocyanate anion within the 1.0×10−4 to 1.0×10−3 M of NaNO3. This was indicated by the constant

Nernstian behavior and the same linearity range obtained at different ionic strength values.

Nevertheless, within the 2.0×10−3 to 3.4×10−3 M NaNO3, the Nernstian slopes were obtained but the

linear range decreased to 1.00×10-6-8.97×10-2M of thiocyanate anion. In higher concentrations

(>1×10−3 M NaNO3, results not showed) it is possible that some of the ion exchange sites in SMZ are

occupied by NO3− ions. As the result it causes a decrease in electrode response compare to the higher

concentrations of the thiocyanate anions. The difference in the intercept values at nitrate

concentrations lower than 1 mM is not significant. The small differences indicate that the sensor is free

from nitrate interference at concentrations of nitrate lower than 1 mM. But, the intercept values at

nitrate concentrations higher than 1 mM are more different and nitrate has much more interference.

The validity of obtained results was performed by statistical methods. By comparing of texp and t0.05,5 =

2.57 at 95% confidence interval it is concluded that there is a random error in experiments [35].


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3.7. Effect of temperature

To investigate the thermal stability of the electrode, calibration curves (Ecell versus log aSCN-)

were constructed at different temperatures covering the range 25–60 ◦C. The electrode exhibited good

Nernstian behavior in the range of 25–45 ◦C. The results are shown in Table 6.

Table 6. Effect of temperature on optimized SMZ-PVC electrode behavior.

Temperature ( C ) Nernstian slope

(mV.decade-1)

Mean of exp. slope

(n=10), mV.decade-1)

texp

25 -59.2 -59.6 ± 0.7 1.81

30 -60.2 -60.0 ± 0.8 0.79

35 -61.2 -61.5 ± 0.5 1.90

40 -62.2 -62.6 ± 0.6 2.10

45 -63.2 -63.6 ± 0.7 1.81

50 -64.2 -65.1 ± 0.6 4.74

55 -65.0 -67.2 ± 1.3 5.35

60 -66.0 -67.9 ± 0.9 6.67

Note: t 0.05, 9 =2.26 (Ref. 35)

Experimental slope calculated from Eq. (3) which is applicable to any temperature:

(3)

where n is the valancy, R, T and F are the universal gas constant, the absolute temperature and

the Faraday constant.

The Nernstian and experimental slope values at each temperature were compared statistically,

showing conformity of the slopes for 10 replicates. The results prove the validation of the obtained

results with the expected values in the temperature range of 25–45 ◦C. However, at temperatures higher

than 45 ◦C the slopes show a significant deviation from the theoretical values. This deviation may be

related to desorption of the surfactant from SMZ and destruction of the electrode surface.

3.8. Response time

The response time of the sensor was defined as t95 for the slope of the calibration curve of

thiocyanate solution when the thiocyanate concentration was increased from 1.0×10−7 to 1.0×10−1 mol

L−1 (Fig. 5), where t95 is the time required for the sensor to reach 95% of the steady state (final signal

that does not change during 60 s) potentiometric value. From the results, the best response time of 5 s

was recorded for membrane having the optimized conditions. After new solutions were exposed to the

electrode the response changed rapidly and remained at a constant value before 5 s of the exposure.


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The potential generated by the membrane (when it’s remained constant in one solution of thiocyanate)

remained constant for 5 min, after which it started slow deviating (−0.5% per min).

Figure 5. Response time of the SMZ-PVC to different concentration of thiocyanate (pH=5.5) in the

range of 1×10−7 to 6×10−1 M (in sequence of “g” to “a” in figure), (Inset: Static response time

of the proposed membrane electrode towards SCN- ions)

The static response time of the membrane electrode thus obtained was


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and interfering ions. The potentiometric selectivity coefficients of the proposed electrode, in the

optimized analytical procedure, are calculated using the following equation [37] and summarized in

Table 7:

(4)

Where is the potentiometric selectivity coefficient, Ei the measured cell potential at

primary ion activity ai (SCN-), Ej the measured cell potential at interference ion activity aj, n and m are

the charges of the primary and interference ion, respectively. R, T and F have their common meaning.

Table 7. Potentiometric selectivity coefficients of various interfering anions (n=4).

Interfering anion logPot

ClOK

4

± Interfering anion logPot

ClOK

4

±

ClO4- -0.9±0.08 CN- -3.1±0.09

I- -1.2±0.05 CO32- -2.6±0.06

IO4- -2.1±0.07 BrO3- -3.0±0.11

ClO3- -2.8±0.07 IO3- -3.1±0.21

C2O42- -2.9±0.09 Cl- -3.3±0.11

NO3- -1.9±0.10 Br- -2.5±0.10

CH3COO− -3.4±0.07 Salicylate -2.4±0.08

ClO4- > I- > NO3

- > IO4- > Salicylate> Br- > CO3

2-> ClO3-> C2O4

2- > BrO3->IO3

- = CN-

As can be seen, of the different anions tested, the largest sensitivity was obtained for

thiocyanate, perchlorate, iodide, nitrate and periodate. a poorer response was observed with salicylate

and the responses to other anions were practically negligible.

The selectivity sequence follows the Hofmeister Series (the selectivity based solely on

lipophilicity of anions) [38], (ClO4− > SCN− > I− >NO3

− >Br− >Cl− >HCO3− >CH3COO

− >SO42−

>HPO42−) although some slight deviations might be interpreted because of some preferential

coordination of this ionophore with certain anions.

3.10. Response characteristics of the electrode

Using the optimized composition and conditions described above, the critical response

characteristics of the electrode were assessed according to IUPAC recommendations [39]. The emf

response of the electrode to varying concentrations of thiocyanate species indicates a linear range from

1.00×10-6 to 1.10×10-1 M (r = 0.9966) (Fig. 6). The average slope of calibration curves based on 14

replicate measurements was −58.9 ± 0.9 mV per decade of SCN− concentration. Comparison of this

calculated value with theoretical Nersntian value (59.2 mV per decade of SCN− concentration) by

statistical (t) parameter (tcalc = 1.87 and t 0.05,13 =2.16) shows that the difference between them is due to


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random errors. On the other hand, the proposed SMZ membrane electrode is applicable to measure

SCN− anion in aqueous solutions. The lower and upper detection limits were 8.33×10−7 and 0.13 M,

respectively.

Figure 6. Potentiometric response of the proposed electrodes toward thiocyanate.

Long term stability of the electrode was investigated by taking the response of the optimized

electrode in a period of 30 days. Potentiometric measurements were performed every day during the

first week and then every three days for the rest of the period. The electrode retained its full activity

during this period. The relative standard deviations (RSD) of a single electrode (within electrode

variation) were determined every 30 s during a period of 0-120s, (n=5). This process was repeated 10

times. The RSD values were between 0.5 and 1.4%. The small relative standard deviations (0.2–1.0%)

and hence good repeatability of the electrodes indicates that the PVC membrane is homogeneous.

The reproducibility of the potentiometric response of the optimized electrode was also

investigated by carrying the measurements on four independent electrodes with the same composition.

These measurements were also performed in 10 replicates. The obtained relative standard deviation

(between electrode variations) was less than 1.5%. The validity of results was investigated by “g”

value statistical test, g-test [35], as:

22

2

2

1

2

... ksss

shighestg

(5)

Where is the variance of each class of determinations. The calculated “g” values for each

case was 0.4793 which are smaller than the critical g value at 95% confidence interval, g0.05,10,4 =


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0.5017 (4 class, 10 replicate in each class) [35] indicating that there is no significant difference

between the behavior of the four independent electrodes and those compositions are homogeneous.

Table 8. Comparison of some characteristics of the proposed electrode with previous SCN− ISEs.

Ref. Slope

(mVdecade−1)

Working

pH range

Linear range (M) DL (M) Ionophore

Proposed

electrode

−58.9 ± 0.9 2.0–10.1 1.00×10-6–1.01×10−1 8.33×10−7 SMZ

[40] -56.3 2.0-8.0 6.0×10-6–1.0×10−2 5. 0×10−6 Tetra-midepyridine

[40] -58.3 2.0-8.0 1.0×10-5–1.0×10−2 5. 0×10−6 Hexa-imidepyridine

[41] -57.0 5.0 9.0×10−7–1.0×10−1 7.0×10−7 [Cu(II)2-BBSTA]

[42] -56.5 3.5–8.5 1.0×10−6–1.0×10−1 7.0×10−7 [Zn(BDSH)]

[43] -58.7 3.0–8.0 1.0×10−5–1.0×10−1 4.0×10−6 Rh[(trpy)(bpy)Cl](PF6)2

[44] -57.6 3.9–9.2 1.0×10−6–1.0×10−1 5.0×10−7 CCT

[44] -58.7 4.3–9.8 1.0×10−6–1.0×10−1 5.7×10−7 NOBP

[45] -52.0 2.0–3.0 3.0×10−5–1.0×10−1 2.5×10−5 Calix[4]arene

Some response characteristics of the proposed electrode compared with other thiocyanate

selective electrodes and the obtained results are summarized in Table 8. As can be seen from Table 8,

in some cases, selectivity coefficient and dynamic linear range that were obtained for the proposed

electrode are superior and comparable to those reported previously for other thiocyanate-selective

membrane electrodes based on variety of different ionophores. It is important to note that in all cases

in Table 8, response time, stability and electrode response independent pH range for the proposed

SMZ-PVC are much better than those reported for other compared electrodes. The SMZ-PVC has

applicability in a wide pH range (2.0–10.1), for acidic to basic solutions. High stability of the electrode

is another advantage of proposed electrode that it in turn causes an increasing of precision. In

investigation of reproducibility and repeatability, the RSD values for our SMZ-PVC (1.3–2.4%) are

smaller than others (2–5%). This shows the SMZ-PVC has a better precision.

3.11. Practical application

The optimized membrane thiocyanate electrode was successfully used for potentiometric

determination of SCN− via Ag+ titrations. Fig. 7 shows the titration curve of 10mL of a 1.0×10−2 M

thiocyanate solution titrated with 1×10-2 M of AgNO3. The titration volume was measured at the

inflection point of the titration curve and a concentration of thiocyanate of 9.7×10-3M was determined.

The new thiocyanate -selective electrode was applied for the determination of thiocyanate in

river and tap water samples with satisfactory results. The analysis was performed by using the standard

addition technique. The results are summarized in Table 9. Good recoveries were obtained in all

samples.


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Figure 7. Potentiometric titration curve for 10 ml of 1.0× 10−2 M SCN- solution with 1.0 ×10−2 M

AgNO3 using the proposed sensor as an indicator electrode (n=5).

Table 9. Determination of thiocyanate in drinking and river water.

Samples Added thiocyanate

(ml/gμ)

Foundet thiocyanate

(ml/gμ)

Recovery(%)

Tap Water of Isfahan - 0 -

1.4 0.21±1.33 95.0

2.3 0.44±2.15 93.5

3.0 0.32±2.92 97.3

5.3 0.45±5.23 98.1

The River Water of

Zayandeh Rood in

Isfahan

- 0 -

10.1 0.23±10.16 100.6

15.3 0.55±15.14 99.3

20.2 0.35±20.0 1 99.1

25.4 0.88±25.12 98.9

4. CONCLUSIONS

To sum up, we have shown that the use of the modified zeolite by cationic surfactant can be

used as suitable ionophores in the development of new thiocyanate-selective PVC membrane

electrodes. These electrodes show a Nernstian response, detection limits of near 8.33×10−7 M and a

low response time of a few seconds in the presence of thiocyanate. The optimized membrane electrode

was used as indicator electrode in potentiometric titrations of thiocyante with silver.


3194

ACKNOWLEDGMENT

We wish to acknowledge the financial support from the Research Council of Islamic Azad University,

Shahreza Branch.

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